1. Field
Carrier aggregation (CA) is a technique that is applicable to such networks as evolved universal terrestrial radio access (E-UTRA) networks (E-UTRAN). E-UTRAN is just one example, as the technique may be used in other kinds of networks. In carrier aggregation, two or more component carriers (CCs) or serving cells can be aggregated in order to support wider transmission bandwidths up to, for example, 100 MHz and for spectrum aggregation. In carrier aggregation, it is possible to configure a user equipment (UE) to aggregate a different number of component carriers or serving cells originating from the same enhanced node B (eNB) and of possibly different bandwidths in the uplink (UL) and the downlink (DL).
2. Description of the Related Art
In releases 8 and 9 of the third generation partnership project (3GPP), without carrier aggregation, a power headroom report is triggered if any of the following occur, see 3GPP specification, 36.321, which is hereby incorporated herein by reference: a timer called “prohibitPHR-Timer” expires or has expired and the path loss has changed more than a predetermined amount, “dl-PathlossChange” dB, since the transmission of a power headroom report when the user equipment has uplink resources for new transmission; a timer called “periodicPHR-Timer” expires; or upon configuration or reconfiguration of the power headroom reporting functionality by upper layers, which is not used to disable the function. The “prohibitPHR-Timer,” “periodicPHR-Timer,” and “dl-PathlossChange,” are configured by a radio resource controller. Similar power headroom report triggering could also be applied to carrier aggregation. When power headroom reporting is triggered, power headroom reports on all the scheduled CC (or serving cells) or configured CC (or configured serving cells) should be reported.
One approach to addressing power headroom reporting is to limit each power headroom report (PHR) to the corresponding component carrier. Although such an approach minimizes changes, because of potential hybrid automatic repeat request (HARQ) retransmission timing differences, the eNB cannot obtain a power headroom report from all component carriers at the same time. Furthermore, one logical channel identifier (LCID) is used per reported component carrier (or serving cell), increasing overhead.
Another option is to allow a cross carrier power headroom report with a cross carrier indicator for each power headroom report to indicate which component carrier (or serving cell) it is referring to. This approach requires one media access control (MAC) sub-header for each individual power headroom report or a “length” field to indicate the length for such variable size payload. Furthermore, there are only 2 R bits in the power headroom report MAC control element (CE) which is not enough to indicate 5 component carriers (or serving cells).